Engineers are developing spark-ignition engines with direct fuel injection (SIDI) which operate using either a stratified combustion charge strategy or a homogeneous combustion charge strategy, depending upon operator demand for power and engine operating conditions. Generally, at high load conditions the SIDI engine is operated with a homogenous charge and at low load conditions the SIDI engine is operated with a stratified charge.
Generally, a spray guided SIDI engine has a fuel injector including an outlet with a multi-port tip to effect multiple cone-shaped spray paths of fuel. In the spray guided SIDI engine, a spark plug and the fuel injector are cooperatively arranged such that a portion of the fuel spray path intersects the spark gap of the spark plug. During stratified combustion modes, fuel interacting with a spark in the spark gap provides ignition of the fuel.
With SIDI engine spark plugs, heat is lost to adjacent surfaces thereby slowing the burning rate of the fuel and thus slowing the growth of a flame kernel. In particular, heat losses to adjacent surfaces can slow the burning rate to a level such that the flame kernel is extinguish, thereby causing a misfire.
A prior art J-gap spark plug 10 is illustrated in FIG. 1. The J-gap spark plug 10 is generally robust to misfire events during homogeneous and warmed-up stratified operating modes. The J-gap spark plug 10 has a single ground electrode 17, a tip insulator 15, a center electrode 13 and an axial spark gap 11 between the center electrode 13 and the ground electrode 17. The axial spark gap 11 is projected away from any metal or insulating surfaces, such that a flame kernel initiated by the arc in the axial spark gap 11 can grow in size without losing heat to adjacent surfaces.
Drawbacks associated with the J-gap spark plug 10 include susceptibility to fouling and misfiring during cold, stratified operating modes. When running cold and stratified, large amounts of fine soot formed in the combustion process condense and adhere to the tip insulator 15. The accumulation of soot on the tip insulator 15 provides a short circuit path for the arc to travel from the center electrode 13 down the tip insulator 15 to a base of the tip insulator 15, rather than across the axial spark gap 11.
A prior art hybrid spark plug 20 is illustrated in FIGS. 2A and 2B. The hybrid spark plug 20 includes three ground electrodes: a J-gap ground electrode 28 and two surface-gap ground electrodes 23. The hybrid spark plug 20 further includes a tip insulator 27 and a center electrode 29. The surface-gap ground electrodes 23 are arranged at 90 degree positions relative to the J-gap ground electrode 28 and diametrically opposing each other (that is, being positioned along a diameter and having an orientation that differs by 180 degrees.) The J-gap ground electrode 28 and the center electrode 29 define an axial spark gap 21. The surface-gap ground electrodes 23 and the center electrode 29 define radial spark gaps 25.
The hybrid spark plug 20 performs well in preventing cold plug fouling but proves marginal in preventing misfires during homogeneous and warmed-up stratified operating modes. When sufficient amounts of soot accumulate on the insulator, misfiring occurs at the axial spark gap 21. By providing the surface-gap ground electrodes 23 in proximity to a surface of the insulator tip 27, the arc is preferentially directed across the radial spark gap 25 preventing a total misfire. Successive spark firings across the radial spark gap 25 burn off the condensed soot on the insulator 27 surface keeping it clean and providing a deterministic path for the arc. The hybrid spark plug 20 exhibits improved cold plug fouling over the J-gap spark plug 10 because carbon accumulation on the insulator tip 27 is burned off by the surface-gap ground electrode 23.
However, because the radial spark gap 25 is very close to the insulator surface, heat loss to the insulator tip 27 is increased. Further, the two surface-gap ground electrodes 23 block fuel streams directed to other parts of a combustion chamber (not shown) of an internal combustion engine (not shown). Therefore, the likelihood of flame-kernel-quenching misfire is increased. Thus, the hybrid spark plug 20 is associated with warmed-up stratified misfiring.